Origin, Recognition, and Importance of Erosional Unconformities in Sedimentary Basins

Abstract

Erosional unconformities of different scales (local to global) are an ubiquitous element of all sedimentary basins. Erosional unconformities of subaerial origin are believed to have been caused by tectonic uplifts and by eustatic sea-level fall. Erosional unconformities of submarine origin may be related to transgression, mass movements, turbidity currents, thermohaline currents, carbonate dissolution, storms, and clastic influx on carbonate shelves. Important criteria for recognizing subaerial unconformities include discordance of dip, karst facies, basal conglomerate, and a major gap in the fossil record. Paleosol horizons, duricrust, and continental deposits, indicative of subaerial exposure, can also be used to define surfaces of potential subaerial unconformities. Submarine unconformities may be recognized by mass-movement deposits, glauconitic minerals, and manganese nodules.

Recognition of unconformities is useful for subdividing stratigraphic units, determining the timing of tectonic activity, interpreting lateral facies relationships, constructing burial and uplift curves, correlating certain stratigraphic boundaries, interpreting sea-level changes, and for reconstructing paleogeography. Erosional unconformities may be important to exploration because they can be used to predict deep-sea turbidite reservoir facies; they can mark upper boundaries of zones of increased porosity (e.g., Statfjord Field, North Sea); they can provide an ideal juxtaposition of reservoir and source rocks (e.g., Prudhoe Bay Field, Alaska); they can act as avenues of hydrocarbon migration (e.g., Maracaibo Basin, Venezuela); they can generate hydrocarbon traps (e.g., Messla Field, Libya); and they can be favorable sites for mineralization (e.g., uranium, aluminum, phosphates, and gold).

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